Acoustic Lens Turns Sound into Sonic Bullets

The manipulation of sound waves has led to critical technologies such as ultrasound imaging. Alessandro Spadoni and Chiara Daraio of the California Institute of Technology have now developed a new type of acoustic lens to make sound waves even more powerful.

Acoustic lenses focus sound in much the same way optical lenses focus light. Instead of using glass and mirrors, the duo designed an acoustic lens using 21 rows of 21 stainless-steel spheres. But instead of firing sound waves at the lens, they actually strike the first sphere in each row “so that we send a compressive wave down each stack or row,” Spadoni explains. The researchers effectively tune the focal point of the lens by changing how hard they strike the lens (affecting the waves’ amplitude) and the size of the spheres (affecting wavelength). The waves are then transmitted into an object, such as a human limb, where they focus down to a point.

The ability to focus could improve ultrasound imaging, according to Spadoni, who described the work online in the April 5 Proceedings of the National Academy of Sciences USA. Even more daring would be “sound bullets” for nonintrusive surgical operations, the researchers say. Acoustic energy can heat tissue, an effect already used in hyperthermia therapy. The Caltech nonlinear acoustic lens could aid in the treatment of tumors by increasing the temperature of cancerous tissue to lethal levels without affecting healthy tissue.